The Layer Thinner bases the contour lines’ thickness on the trace depth as determined by the Contour Contrast Function Levels shader.
In the following illustration, the glass pane is semi-transparent. The shader detects that situation and makes the lines thinner when they are drawn inside the glass pane.
The factor for thinning the contour at each layer.
For example, to get contours that are 2 percent wide when the material is on top, 1 percent wide when the material is behind one other (semitransparent) material, 0.5 percent wide when the material is behind two materials, and so on, set Width (%) to 2.0 and Depth Factor to 0.5.